There has been widely used a method of analysis using gas concentration in oil as a maintenance method for an oil-filled electric apparatus. For example, when an abnormality occurs inside an oil-filled electric apparatus such as a transformer which is in operation, there is used a so-called gas pattern method (Non Patent Document 1) or the like in which the gas concentration in oil of a sample oil which is taken from the oil-filled electric apparatus is measured, a detail and a location of the abnormality are estimated from the type of gas in oil and the concentration pattern thereof on the basis of past cases.
However, in such an analysis method, since a diagnostic specialist performs the estimation from the measured data on the basis of past cases by using his/her knowledge obtained from experience as a criterion for determination, a result of the diagnosis may vary due to a difference in the level of knowledge or experience between diagnostic specialists. Further, there is a limit on the number of diagnoses which each diagnostic specialist can deal with, and it takes time to deal with a huge number of diagnoses. Therefore, replacement or repair of an oil-filled electric apparatus may not be able to be performed at an appropriate time. Further, it requires a considerable amount of time to train a diagnostic specialist of a certain level, and a heavy burden is therefore imposed on skilled diagnostic specialists in the present circumstances.
In order to improve such a situation, an attempt for performing diagnosis using a machine has been made (Non Patent Document 2). In Non Patent Document 2, there is described an example in which a fuzzy decision tree based on ID3 algorithm is applied to analysis and diagnosis of gas in oil. In this example, when generating a fuzzy decision tree, measured values of the concentrations of a plurality of gas species are employed without change to form a fuzzy set and generate discretized data converted into a plurality of discretized attributes. However, when the measured values of the gas concentrations are used without change in this manner, relationships among the plurality of gas species are not considered. Therefore, this method diverges from a determination method by a diagnostic specialist, and therefore has a limit on accuracy as diagnosis using a machine.
Further, although various proposals have been made regarding a diagnosis method and a prediction method using a decision tree (see Patent Documents 1 and 2, for example), a method using a decision tree that can be applied to analysis of gas in oil has not yet been proposed, and a development of such a method has therefore been expected.